Capacitive Position Sensor

ABSTRACT

A capacitive position sensor has a two-layer electrode structure. Drive electrodes extending in a first direction on a first plane on one side of a substrate. Sense electrodes extend in a second direction on a second plane on the other side of the substrate so that the sense electrodes cross the drive electrodes at a plurality of intersections which collectively form a position sensing array. The sense electrodes are provided with branches extending in the first direction part of the way towards each adjacent sense electrode so that end portions of the branches of adjacent sense electrodes co-extend with each other in the first direction separated by a distance sufficiently small that capacitive coupling to the drive electrode adjacent to the co-extending portion is reduced. Providing sense electrode branches allow a sensor to be made which has a greater extent in the first direction for a given number of sense channels, since the co-extending portions provide an interpolating effect. The number of sense electrode branches per drive electrode can be increased which allows a sensor to be made which has ever greater extent in the first direction without having to increase the number of sense channels.

BACKGROUND OF THE INVENTION

The invention relates to capacitive position sensors. More particularlythe invention relates to 2-dimensional capacitive position sensors ofthe type based on capacitive proximity sensing techniques. Such sensorsmay be referred to as 2-dimensional capacitive transducing (2DCT)sensors. 2DCT sensors are based on detecting a disturbance in acapacitive coupling of sensor electrodes caused by the proximity of apointing object. A measured location for the disturbance corresponds toa measured position for the pointing object.

U.S. Pat. No. 6,452,514, U.S. Pat. No. 7,148,704 and U.S. Pat. No.5,730,165 disclose prior art capacitive touch sensors.

2DCT sensors are typically actuated by a human finger, or a stylus.Example devices include touch screen and touch sensitivekeyboards/keypads, e.g. as used for controlling consumer electronicdevices/domestic appliances, and possibly in conjunction with anunderlying display, such as a liquid crystal display (LCD), or cathoderay tube (CRT). Other devices which may incorporate 2DCT sensors includepen-input tablets and encoders used in machinery for feedback controlpurposes, for example. 2DCT sensors are capable of reporting at least a2-dimensional coordinate, Cartesian or otherwise, related to thelocation of an object or human body part, by means of a capacitancesensing mechanism.

Devices employing 2DCT sensors have become increasingly popular andcommon, not only in conjunction with personal computers, but also in allmanner of other appliances such as personal digital assistants (PDAs),point of sale (POS) terminals, electronic information and ticketingkiosks, kitchen appliances and the like. 2DCT sensors are frequentlypreferred to mechanical switches for a number of reasons. For example,2DCT sensors require no moving parts and so are less prone to wear thantheir mechanical counterparts. 2DCT sensors can also be made inrelatively small sizes so that correspondingly small, and tightly packedkeypad arrays can be provided. Furthermore, 2DCT sensors can be providedbeneath an environmentally sealed outer surface/cover panel. This makestheir use in wet environments, or where there is a danger of dirt orfluids entering a device being controlled attractive. Furthermore still,manufacturers often prefer to employ interfaces based on 2DCT sensors intheir products because such interfaces are often considered by consumersto be more aesthetically pleasing than conventional mechanical inputmechanisms (e.g. push-buttons).

WO 2009/027629, published on 5 Mar. 2009, describes a capacitive touchsensor comprising a dielectric panel overlying a drive electrode withtwo sense electrodes. One of the sense electrodes is positioned to beshielded from the drive electrode by the first sense electrode, so thatthe first sense electrode receives the majority of the charge coupledfrom the drive electrode and the second sense electrode primarilyregisters noise. A sensing circuit including two detector channels isconnected to the first (coupled) and second (noise) sense electrodes toreceive signal samples respectively. The sensing circuit is operable tooutput a final signal obtained by subtracting the second signal samplefrom the first signal sample to cancel noise.

However, the methods described above increase the size and thickness,and may decrease the resolution of a device incorporating a displayscreen with a 2DCT sensor when it is more fashionable and desirable toproduce smaller devices. Furthermore, additional steps are requiredduring manufacture and as a result there is an increased cost due tofurther components being needed.

European patent EP 1821175 describes an alternative solution to reducethe noise collected on a 2DCT touch sensor. EP 1821175 discloses adisplay device with a touch sensor which is arranged so that the twodimensional touch sensor is overlaid upon a display device to form atouch sensitive display screen. The display device uses an LCDarrangement with vertical and horizontal switching of the LCD pixels.The touch sensing circuit includes a current detection circuit, a noiseelimination circuit as well as a sampling circuit for each of aplurality of sensors, which are arranged to form the two-dimensionalsensor array. The current detection circuit receives a strobe signal,which is generated from the horizontal and vertical switching signals ofthe LCD screen. The strobe signal is used to trigger a blanking of thecurrent detection circuit during a period in which the horizontalswitching voltage signal may affect the measurements performed by thedetection circuit.

WO 2009/016382, published on 5 Feb. 2009, describes a sensor used toform a two dimensional touch sensor, which can be overlaid on a liquidcrystal display (LCD) screen. As such, the effects of switching noise onthe detection of an object caused by a common voltage signal of the LCDscreen can be reduced. The sensor comprises a capacitance measurementcircuit operable to measure the capacitance of the sensing element and acontroller circuit to control charging cycles of the capacitancemeasurement circuit. The controller circuit is configured to producecharging cycles at a predetermined time and in a synchronous manner witha noise signal. For example, the charge-transfer cycles or ‘bursts’ maybe performed during certain stages of the noise output signal from thedisplay screen, i.e. at stages where noise does not significantly affectthe capacitance measurements performed. Thus, the sensor can be arrangedto effectively pick up the noise output from a display screen andautomatically synchronize the charge-transfer bursts to occur duringstages of the noise output cycle.

FIG. 21 of the accompanying drawings illustrates schematically arepresentative portion of the prior art electrode pattern of U.S. Pat.No. 6,452,514 or its equivalent WO 00/44018, published on 27 Jul. 2000.A plurality of drive electrodes X1, X2, X3 and X4 extending rowwise arearranged with a plurality of sense electrodes Y1, Y2, Y3 and Y4extending columnwise, the intersections or crossings between X and Yelectrodes forming a matrix or grid of sensing points or areas 220. Itwill be understood the X and Y electrodes do not literally intersect,but are offset in the vertical or Z direction, orthogonal to the planeof the drawing, being separated by a dielectric layer—typically asubstrate panel which bears the X electrodes on one side and the Yelectrodes on the other side. Each crossed electrode area 220 acts as akey so that the presence of a body such as a user's finger is detectedas a result of a change in an amount of charge which is transferredbetween the two electrodes at the key location. With this arrangement,each of the electrodes X1, X2, X3 and X4 are driven with a drive circuit118 via connections 105 and the other electrodes Y1, Y2, Y3 and Y4 areconnected to a charge measurement circuit 118 via sense channels 116which detects an amount of charge present at each of the sensing areas220. It will be appreciated that for simplicity all of the controlcircuitry has been included in a single circuit 118. Such twodimensional capacitive transducing (2DCT) sensors are typically usedwith devices which include touch sensitive screens or touch sensitivekeyboards/keypads which are used in for example in consumer electronicdevices and domestic appliances. The 2DCT is of the so-called “active”or “mutual” type, in which proximity of an object is sensed by thechanges induced in coupling between a drive electrode and one or moreadjacent sense electrodes.

In the above 2DCT sensor, interpolation is used to determine thelocation of an object or finger adjacent the sensor. This is done byusing the signals from the sense area being touched and the neighboringsense areas in a linear interpolation algorithm. However, for aninterpolation to be accurate the electric field between adjacent driveelectrodes should be linear or at least known. If the electrodes areplaced close together it can be assumed that the electric filed betweentwo electrodes is linear. That is to say that as you move away from anelectrode, the field reduces in a linear fashion.

As the size of devices that use 2DCT sensors is increased, larger area2DCT sensors are required. To increase the area of the 2DCT sensor whilekeeping the same resolution and accuracy (i.e. avoid using a non-linearinterpolation method) the number of drive and sense electrodes could beincreased. However, this means that the number of connections requiredfrom the control circuits is increased which in turn results in moreexpensive control circuits and increased acquisition times, since theacquisition of signals from each of the sensing areas typically needs tobe carried out at least partially in series, since not all sensing areascan be polled simultaneously owing to restrictions on the number ofdrive and sense lines, and controller channels, i.e. chip pins.

FIG. 22 of the accompanying drawings illustrates schematically arepresentative portion of the prior art electrode pattern US2008/0246496, published on 9 Oct. 2008. The figure illustrates a patternof electrodes comprising longitudinal (bar) drive electrodes 152. Thedrive electrodes 152 are coupled via drive channels 158 and 160 to acontroller (not shown in the figure). Each drive channel supplies drivesignals to the group of four drive electrodes 152. The drive electrodes152 are each connected to one another by a chain or row of resistors 170having the same value. Alternatively, a single resistive strip could beused (not shown in figure). When operated the grouped drive electrodeswill receive a different value drive signal. For example, when drivechannel 160 is connected to a drive signal and drive channel 158 isconnected to ground, the electrode connected directly to drive channel160 will receive the applied signal value, the drive electrode belowwill receive two thirds of the applied signal value and the driveelectrode below that will receive a third of the applied signal value.In the example described above, the fourth electrode connected directlyto the drive channel 158 in the figure will be connected to ground.However, the above method can be repeated with drive channel 158 beingconnected to a drive signal and drive channel 160 being connected toground. This effectively, allows four drive electrodes to be drivenusing only two drive channels. The arrangement shown in the figure canbe repeated, and expanded to include more intermediate drive electrodeswith respective resistors. However, the method described above is onlysuitable for the drive electrodes and is not transferable to the senseelectrodes. The sense electrodes shown in the figure are interleavedwith adjacent drive electrodes on a single surface. However, it will beappreciated that the drive electrodes shown in the figure could also beused for two-layer or dual layer designs.

It would therefore be desirable to provide an electrode pattern for amutual capacitive or active type 2DCT sensor that can be used to allowthe size of the overall sensitive area to be increased without needingto introduce more sense channels.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided acapacitive position sensor comprising: a plurality of drive electrodesextending in a first direction on a first plane; a plurality of senseelectrodes extending in a second direction on a second plane offset fromthe first plane so that the sense electrodes cross the drive electrodesat a plurality of intersections which collectively form a positionsensing array; wherein the sense electrodes have branches extending inthe first direction part of the way towards each adjacent senseelectrode so that end portions of the branches of adjacent senseelectrodes co-extend with each other in the first direction separated bya distance sufficiently small that capacitive coupling to the driveelectrode adjacent to the co-extending portion is reduced.

In one embodiment, for each drive electrode there is one set of senseelectrode branches providing co-extending portions that occupy a regionin between adjacent sense electrodes in the first direction. Providingsense electrode branches allow a sensor to be made which has a greaterextent in the first direction for a given number of sense channels.

In other embodiments, for each drive electrode there are multiple setsof sense electrode branches that are offset from each other in thesecond direction, the multiple sets providing respective co-extendingportions extending over different respective regions in the firstdirection. Increasing the number of sense electrode branches per driveelectrode allows a sensor to be made which has ever greater extent inthe first direction without having to increase the number of sensechannels.

The sense electrodes are separated from each other in the firstdirection by a distance P_(sense) and the drive electrodes are separatedfrom each other in the second direction by a distance P_(drive),wherein: P_(sense)/P_(drive)=2 m±1, where ‘m’ is the number of sets ofsense electrode branches per drive electrode. The drive electrode pitchP_(drive) is preferably of comparable dimension, or smaller, to thetouch size of the touching object for which the sensor is designed. Thetouching object for which the sensor is designed may be a finger, e.g.of touch size 8-10 mm diameter. A stylus could also be used.

In plan view each drive electrode covers an area that fully encloses itsassociated sense electrode branches. In other words the ‘footprint’ ofthe sense electrodes lie within their associate drive electrode, or theperiphery of the drive electrode lies outwardly beyond the senseelectrodes associated therewith in at least the second direction andpreferably also the first direction.

The drive electrodes preferably substantially entirely cover the firstplane with individual ones of the drive electrodes being separated fromneighboring drive electrodes by small gaps, wherein the gaps arepreferably dimensioned to be sufficiently small to be invisible oralmost invisible. The gaps are preferably less than around 100micrometers, for example with ITO drive electrodes. Gap values of lessthan 90, 80, 70, 60 or 50 micrometers may also be preferred. With someelectrode materials, such as PET, it may be difficult to fabricate suchsmall gaps, so in some instances the gaps are preferably less thanaround 250, 200 or 150 micrometers

The drive and sense electrodes may be the only electrode layersprovided, a two-layer electrode construction leading to improved opticaltransmission for transparent embodiments such as used fortouch-sensitive displays, thinner overall construction, and lower cost.

An important combination is the above-defined capacitive touch sensorwith a display module. The display module, for example an LCD or OLEDdisplay panel, will typically by arranged below the first layer anddistal the touch surface so that from top to bottom, or outside toinside the device, the components will be—dielectric layer the uppersurface of which will be the touch surface—layer 2—substrate—layer1—display panel, with the display panel being inside the device housingor outer shell. In a display application, the electrodes will likely bemade of ITO.

In some embodiments, each drive and/or sense electrode is made of acontinuous sheet of electrically conductive material, such as ITO or ametal. In other embodiments, each drive and/or sense electrode is madeof a mesh or filigree pattern of interconnected lines of highlyconductive material which collectively define each electrode. Stillfurther embodiments use continuous sheets for one of the electrode typesand meshes for the other electrode type. In the mesh approach, theinterconnected lines preferably have a sufficiently small width so as tobe invisible or almost invisible. They can then be made of material thatis not inherently invisible, e.g. a metal such as copper, but stillremain practically invisible.

The invention can be implemented to form a Cartesian ‘xy’ grid of touchsensor locations. In particular, the drive electrodes can extend in afirst linear direction and the sense electrodes in a second lineardirection transverse to the first linear direction so that the pluralityof intersections form a grid pattern, for example a square, diamond orrectangular grid. The invention can also be implemented to form a polar‘rθ’ grid, wherein the drive electrodes extend arcuately and the senseelectrodes extend radially so that the plurality of intersections lie onone or more arcuate paths.

A further aspect of the invention relates to a touch sensitive panel fora capacitive touch sensor, the touch sensitive panel having a pluralityof drive electrodes arranged on one side of a substrate in a first layerand a plurality of sense electrodes arranged on the other side of thesubstrate in a second layer so that the sense electrodes cross the driveelectrodes at a plurality of intersections offset from each other by thethickness of the substrate, wherein the drive electrodes substantiallyentirely cover the first layer with individual ones of the driveelectrodes being separated from neighboring drive electrodes by smallgaps.

According to a second aspect of the invention there is provided a touchsensitive panel for a capacitive touch sensor, the touch sensitive panelhaving a plurality of drive electrodes arranged on one side of asubstrate in a first layer and a plurality of sense electrodes arrangedon the other side of the substrate in a second layer so that the senseelectrodes cross the drive electrodes at a plurality of intersectionsoffset from each other by the thickness of the substrate, wherein thesense electrodes have branches extending in the first direction part ofthe way towards each adjacent sense electrode so that end portions ofthe branches of adjacent sense electrodes co-extend with each other inthe first direction separated by a distance sufficiently small thatcapacitive coupling to the drive electrode adjacent to the co-extendingportion is reduced.

According to a third aspect of the invention there is provided a methodof sensing position of an actuation on a two-dimensional position sensoraccording to the first aspect of the invention, the method comprising:

applying drive signals to each of the plurality of drive electrodes;measuring sense signals received from each of the plurality of senseelectrodes representing a degree of capacitive coupling of the drivesignals between the drive electrodes and each group of the senseelectrodes;determining the position in the first direction by an interpolationbetween sense signals obtained from each of the plurality of senseelectrodes; anddetermining the position in the second direction by an interpolationbetween sense signals obtained by sequentially driving each of theplurality of drive electrodes with respective drive signals.

According to an alternative formulation of the invention, there isprovided a capacitive sensor having an electrode pattern comprising aplurality of sense electrodes generally extending in a y directionacross a sensing area and spaced apart in an x direction; wherein theextent in the y direction of each of the sense electrodes is hereinreferred to a spine; wherein each of the sense electrodes furthercomprises a plurality of extents spaced apart in the y direction hereinreferred to as first-branches that extend from the spine in the xdirection and a −x direction opposing the x direction, whose extent fromthe spine in the second and −x direction is not more than the spacingbetween adjacent spines; and wherein the first-branches of each of thesense electrodes coextend over the same portion of the sensitive area asthe first-branches of adjacent spines.

The electrode pattern may further comprise a plurality of driveelectrodes extending in the x direction and interleaved in the ydirection; wherein each of the drive electrodes extends in the first andx direction over the same portion of the sensing area as thefirst-branches of each of the sense electrodes.

The drive and sense electrodes may be disposed on opposing surfaces of asubstrate.

The drive and sense electrodes may be disposed on a surface of differentsubstrates.

The electrode pattern may further comprise a plurality of second-,third- or fourth-branches interleaved with the first-branches, whereinthe coextension of branches from adjacent spines is offset from eachother.

According to another aspect of the present invention there is provided atwo-dimensional sensor comprising the electrode pattern, wherein thesensor may further comprise a controller comprising a drive unit forapplying drive signals to the drive electrodes, and a sense unit formeasuring sense signals received from each of the respective senseelectrodes representing a degree of capacitive coupling of the drivesignals between the drive electrodes and each of the sense electrodes.

The controller may further comprise a processing unit for calculating aposition of an interaction with the sensitive area from an analysis ofthe sense signals obtained by applying drive signals to the driveelectrodes.

The processing unit may be operable to determine position in the xdirection by an interpolation between sense signals obtained from eachof the plurality of sense electrodes.

The processing unit may be operable to determine position in the ydirection by an interpolation between sense signals obtained bysequentially driving each of the plurality of drive electrodes withrespective drive signals.

According to another aspect of the present invention there is provided amethod of sensing position of an actuation on a two-dimensional positionsensor comprising: an electrode pattern comprising a plurality of senseelectrodes generally extending in a y direction across a sensing areaand spaced apart in an x direction, wherein the extent in the ydirection of each of the sense electrodes is herein referred to a spine,wherein each of the sense electrodes further comprises a plurality ofextents spaced apart in the y direction herein referred to asfirst-branches that extend from the spine in the x direction and a −xdirection opposing the x direction, whose extent from the spine in thesecond and −x direction is not more than the spacing between adjacentspines, and wherein the first-branches of each of the sense electrodescoextend over the same portion of the sensitive area as thefirst-branches of adjacent spines; a plurality of drive electrodesextending in the x direction and interleaved in the y direction; whereineach of the drive electrodes extends in the first and x direction overthe same portion of the sensing area as the first-branches of each ofthe sense electrodes; the method comprising: applying drive signals toeach of the plurality of drive electrodes; measuring sense signalsreceived from each of the plurality of sense electrodes representing adegree of capacitive coupling of the drive signals between the driveelectrodes and each group of the sense electrodes; determining theposition in the x direction by an interpolation between sense signalsobtained from each of the plurality of sense electrodes; and determiningthe position in the y direction by an interpolation between sensesignals obtained by sequentially driving each of the plurality of driveelectrodes with respective drive signals.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings.

FIG. 1A shows a side view of a two-electrode layer capacitive touchscreen according to an embodiment of the present invention;

FIG. 1B shows a perspective view of a two-electrode layer capacitivetouch screen according to an embodiment of the present invention;

FIG. 1C shows a side view of a two-electrode layer capacitive touchscreen according to another embodiment of the present invention;

FIG. 1D shows a side view of a two-electrode layer capacitive touchscreen according to another embodiment of the present invention;

FIG. 1E shows a side view of a two-electrode layer capacitive touchscreen according to an embodiment of the present invention;

FIG. 2A shows an electrode pattern of drive electrodes with resistiveelements according to an embodiment of the invention;

FIG. 2B shows a portion of the electrode pattern shown in FIG. 2A with ameander pattern of electrode material;

FIG. 2C shows a portion of the electrode pattern shown in FIG. 2A withscreen printed resistors;

FIG. 2D shows a portion of the electrode pattern shown in FIG. 2A withdiscrete resistors;

FIG. 3 shows a portion of the electrode pattern shown in FIG. 2B.

FIG. 4 shows a portion of the electrode pattern of drive electrodesaccording to an embodiment of the invention;

FIG. 5A shows a portion of the electrode pattern shown in FIG. 2A;

FIG. 5B shows a typical finger tip;

FIG. 6 shows an electrode pattern of drive electrodes according to anembodiment of the invention;

FIG. 7A shows an electrode pattern of sense electrodes according to anembodiment of the invention;

FIG. 7B shows a two-electrode layer capacitive touch screen according toan embodiment of the present invention with drive and sense unitsconnected via channels to a controller;

FIG. 8A shows schematically in plan view a portion of the electrodepattern shown in FIG. 7A with infilling electrodes;

FIG. 8B is a cross-section through a part of FIG. 8A illustratingcapacitive paths between infilling electrodes and an X electrode;

FIG. 9 shows hand-shadow caused by a proximate location of the palm,thumb, wrist etc to a touch screen when the user touches with a finger;

FIG. 10 shows a portion of an electrode arrangement of sense electrodes;

FIG. 11 shows a two-electrode layer capacitive touch screen according toanother embodiment of the present invention;

FIG. 12 shows a position sensor array according to an embodiment of theinvention;

FIG. 13A shows a side on view the position sensor array according to anembodiment of the invention;

FIG. 13B shows a side on view the position sensor array according to anembodiment of the invention;

FIG. 13C shows a side on view the position sensor array according to anembodiment of the invention;

FIG. 14A schematically shows a circuit which may be used to measure thecharge transferred from a driven one of the drive electrodes to thesense electrodes;

FIG. 14B shows schematically the timing relationships of operation ofthe circuit of FIG. 14A;

FIG. 15A shows a position sensor array according to another embodimentof the invention;

FIG. 15B shows an expanded portion of the position sensor shown in FIG.15A, showing the number of exposed branch edges;

FIG. 16 shows a position sensor array according to another embodiment ofthe invention;

FIG. 17 shows a position sensor array according to another embodiment ofthe invention;

FIG. 18 schematically shows a portable personal computer incorporating asensor according to an embodiment of the invention;

FIG. 19 schematically shows a washing machine incorporating a sensoraccording to an embodiment of the invention;

FIG. 20 schematically shows a cellular telephone incorporating a sensoraccording to an embodiment of the invention;

FIG. 21 illustrates schematically a representative portion of a priorart position sensor;

FIG. 22 illustrates schematically a representative portion of a priorart position sensor.

DETAILED DESCRIPTION

Described herein is a two-electrode layer construction for a capacitivetouch screen or 2DCT sensor.

FIGS. 1A and 1B are schematic drawings in side view and perspective viewof a two-electrode layer construction for a capacitive touch screen or2DCT sensor. The layers 101 can generally be made of any conductivematerial and the layers can be arranged to oppose each other on twosides of any isolating substrate 102 such as glass, PET, FR4 etc. Thethickness of the substrate 103 is non critical. Thinner substrates leadto higher capacitive coupling between the layers which must be mitigatedin the control chip. Thicker substrates decrease the layer to layercoupling and are generally more favorable for this reason (because themeasured change in capacitance is a larger fraction of thelayer-to-layer capacitance so improving signal-to-noise ratio). Typicalsubstrate thickness' range from 10's to 100's of μm. Furthermore it willappreciated that a dielectric or isolating layer may be disposedoverlying the two-electrode layer construction on Layer 2 to prevent anobject adjacent the 2DCT sensor making contact with the surface of thelayers. This isolating layer might be a glass or plastics layer.

FIG. 1C shows the side view of an alternative arrangement to thetwo-electrode layer construction for the capacitive touch screen or 2DCTsensor shown in FIG. 1A according another embodiment of the presentinvention. In FIG. 1C the layers 101 are disposed on the same surface ofthe isolating substrate 102, separated by an isolation layer 108. Anadditional dielectric or isolating layer 104 is disposed on theelectrodes layers to prevent an object adjacent the 2DCT sensor makingcontact with the layers surface.

FIG. 1D shows the side view of an alternative arrangement to thetwo-electrode layer construction for the capacitive touch screen or 2DCTsensor shown in FIG. 1A according another embodiment of the presentinvention. In FIG. 1D the layers 101 are disposed on the same surface ofthe isolating substrate 102, separated by an isolation layer 108.However, the electrode layers 101 are disposed on the surface of theisolating substrate that is farthest from the touch surface 106. Adisplay panel 100 is also shown (with hatching) arranged below thesubstrate 102 that bears the electrode layers 101. It will be understoodthat the display panel in combination with the touch sensor make a touchscreen. A display panel could also be fitted to an arrangement as shownin FIG. 1C above.

FIG. 1E shows the side view of an alternative arrangement to thetwo-electrode layer construction for the capacitive touch screen or 2DCTsensor shown in FIG. 1A according another embodiment of the presentinvention. In FIG. 1E each of the layers 101 are disposed on a surfaceof two different isolating substrates 102. The two isolating substratesare brought together such that the two electrode layers 101 areseparated from the touch surface 106 and are separated by one of theisolating substrates. A display panel could also be fitted to anarrangement as shown in FIG. 1E.

FIG. 2A shows an electrode pattern of drive electrodes with resistiveelements according to an embodiment of the invention. Layer 1 is thelayer farthest from the touch surface. On Layer 1 is an array oftransmitting electrodes as shown in FIG. 2A. The electrodes 201 arearranged as a series of solid bars running along a first axis 202 or a ydirection. A subset of the bars 203 is connected to the control chip sothat they can be driven as the transmitter in the transmit-receivearrangement described above. The driven bars 203 include the outer mostbars and then an even gap 204 between the remaining driven bars. Theintermediate bars 205 are connected using resistive elements 206 in achain 210, the ends of the chain being connected to two adjacent driven203 bars. The driven bars 203 will be referred to as driven-X-bars andthe resistively connected bars 205 will be referred to asresistive-X-bars.

FIGS. 2B, 2C and 2D show three different ways in which to form theresistive elements 206. Namely, the resistive elements 206 can be formedusing the intrinsic resistance of the electrode material itself in a“meandered” pattern 207 at the edge of the touch screen (see FIG. 2B),or can be screen printed resistive material 208 at the edge (see FIG.2C), or can be physical discrete resistors 209 either at the edge of thepattern (see FIG. 2D) or on a separate circuit. The latter optionincreases the interconnecting wiring substantially but can beadvantageous in some designs.

The resistive chain 210 is used to act as a classic potential divider,such that the amplitude of the transmit signal is progressivelyattenuated between one driven-X-bar and the adjacent driven-X-bar. Theset of driven and resistive bars so described will be referred to as a“segment” 211. Using this chain, if say driven-X-bar #1 303 is drivenwith a pulse train 305 relative to 0V 306 with a peak-to-peak voltage V307, and driven-X-bar #2 304 is driven to 0V, then resistive-X-bars inbetween these two will be ratiometrically attenuated.

FIG. 3 shows a portion of the electrode pattern shown in FIG. 2B inwhich example, if there were 2 resistive-X-bars 205 and the resistordivider chain 210 is constructed of equal valued elements R 308, thenthe resistive-X-bar #1 301 will have a peak-to-peak voltage of 0.66666Vand resistive-X-bar #2 will have a peak-to-peak voltage of 0.33333V.This has the effect of progressively weakening the electric fieldemitted from these resistive electrodes and so forms an interpolatingeffect for the capacitive changes within the segment betweendriven-X-bars. Hence, the linearity of the capacitive changes whenmoving within a segment is improved. Operating without resistive-X-barsis possible but the linearity is poor because the electric field decaysover distance in a strongly non-linear fashion. By introducing evenlyspaced resistive emitters emitting at an amplitude that is a lineardivision from the associated driven-X-bar, the field tends to “fill in”and form a better approximation to a linear system.

In the forgoing description Layer 1 is a pattern of transmit-electrodes,which may also be referred to as drive electrodes. The electrode patternof Layer 1 may also be refereed to as x-electrodes. The drive electrodesinclude the driven-X-bars 203 and the intermediate X bars 205 orresistive-X-bars. Furthermore, the driven or drive electrodes aredefined as being made up of outer most driven-X-bars 203 andintermediate X bars or resistive-X-bars 205 connected using resistiveelements 206 in a chain 210. The outer most X bars are referred to asdriven-X-bars 203. However, it will be appreciated that all of theX-bars might be driven X-bars without using resistive elements.

Typical resistive elements 206 have resistive values ranging from a fewKΩ up to high 10's of KΩ. Lower values require more current (and henceenergy) to drive from the control chip but allow faster capacitivemeasurements as they have lower time constants and hence can be chargedand discharged faster. Higher values require less current (and henceenergy) to drive but have higher time constants and hence must becharged and discharged more slowly. Larger values also help to make anyresistance build up in interconnecting wiring contribute a smallervoltage drop to the emitted field strength from the X bars, and hencemake for a more efficient system. For this reason, generally highervalues are preferred.

Another key reason to include the resistive-X-bars is that it makes thesegment scalable, i.e. by adding more resistive-X-bars the segment canbe made larger. This is at the expense of spatial resolution; thesegment uses the same two driven-X-bars and hence the resolution of themeasurement must be fundamentally the same, but the segment is nowspread across a larger region and so spatially the resolution degrades.Making the segment scalable means that fewer driven-X-bars are neededand hence fewer connections to the control chip. By balancing thetrade-off between spatial resolution and connection cost/complexity anoptimal solution may be found for each design.

Overall, the bars in Layer 1 can be seen to be substantially areafilling; almost all of the surface area is flooded with electrode. Thegaps between the bars 205 can be made arbitrarily small and indeed, thesmaller the better from a visibility point of view. Making the gapslarger than around 100 μm is non-ideal as this leads to increasedvisibility of the gap to the human eye and a key goal is often to tryand make an invisible touch screen. A larger gap also tends to increasethe possibility of a significant fringing electric field near the gap toelectrodes in Layer 2 which will lead to worsening non-linearity. Gapsof a few 10's of micrometers are common as they are almost invisible andcan be easily mass-produced, for example gaps of between 20 and 50micrometers.

FIG. 4 shows a portion of the electrode pattern of drive electrodesaccording to an embodiment of the invention. Referring to FIG. 4, it isalso desirable to use a gap with a small up/down wave pattern 401between driven 402 and resistive-X-bars 403 as this helps to disguisethe gap when viewed through Layer 2 with the added effect of theparallax caused by the substrate thickness. Various patterns can be usedto help disguise the gap when viewed in this way, for example a sinewave, triangle wave or square wave could be used. The frequency andamplitude are chosen to help break up the otherwise long linear gap whenviewed through the complex but regular pattern in Layer 2. The amplitudemust be minimized to avoid errors in the reported touch coordinate.

FIG. 5A shows a portion of the electrode pattern shown in FIG. 2A.

FIG. 5B shows a typical finger tip.

The electrode bars (both types) are generally designed so that they havea fundamental pitch of around 8 mm or less, as shown in FIG. 5Apreferably 5 mm. This is in recognition that, as shown in FIG. 5B, atypical finger touch 501 creates a generally circular region 502(illustrated in FIG. 5B with hashing) of around 8 to 10 mm in diameterand so matching the electrode pitch to the touch size optimizes theinterpolating effect of the touch. Making the pitch of the electrodeslarger than 8 mm can start to lead to distinct non-linearity in theresponse as the interpolation is well below ideal. In essence, by makingthe electrode bars too wide, as the touching finger moves perpendicularto the bars its influence tends to “saturate” over one electrode beforeit starts to interact with the next electrode to any significant degree.When the pitch is optimized, the finger will cause a steadily reducinginfluence on one bar while already starting to create a well balancedincrease on the neighboring bar, with the peak influence being spatiallyquite distinct i.e. steady increase immediately followed by steadydecrease with no appreciable transition distance from increase todecrease (or vice-versa).

FIG. 6 shows an electrode pattern of drive electrodes according to anembodiment of the invention. Referring to FIG. 6 the driven-X-bars atthe outer edges of Layer 1 601 are made to be half the width of allother bars 602. The overall design is in essence several identicalconcatenated segments 603, and the driven-X-bars on the inside of thelayer 604 are also half width but are butted up to the neighboringsegment with its half width outer bar, so driven-X-bars internal to thepattern appear to be full width. FIG. 6 shows the virtual division ofthe internal bars 604 with a dashed line; in practice of course the bars604 are one-piece. Having the pattern at its outer two edges withhalf-width bars improves the overall linearity; if the pattern wereinfinite then the linearity would be perfect in this regard, but ofcourse the pattern must end and hence there is a natural non-linearityat the edges.

FIG. 7A shows an electrode pattern of sense electrodes according to anembodiment of the invention. Layer 2 is the layer nearest to the touchsurface. Referring to FIG. 7A in its simplest form, the electrodes onLayer 2 are a uniformly spaced series of narrow lines running along asecond axis at nominally 90 degrees to the first axis used in Layer 1herein referred to as an x direction. That is to say that the Layer 1 ordrive electrodes cross the Layer 2 or sense electrodes. The electrodeson Layer 2 are referred to as sense electrodes, y-electrodes, Y lines orreceive electrodes. They are arranged to lie directly and completelyover the area 703 occupied by the X bars underneath. The spacing betweenthe bars has a similar influence on the linearity as does the spacing ofthe X bars. This means that the Y lines need to be spaced with a pitchof 8 mm or less 704, preferably 5 mm for best intrinsic linearity. In asimilar way to the Layer 1 with its half-width outer X bars, the gapfrom the edge of the Layer 2 pattern to the first line is half of thispitch 705 to improve the linearity. The width of the Y lines 706 isimportant. They need to be narrow enough so that they are not easilyvisible to the human eye, but wide enough that they have a resistance(at their “far-end”) that is low enough to be compatible with capacitivemeasurements. Narrower is also better as far as noise immunity isconcerned because the surface area of the Y line has a direct influenceof on how much electrical noise can be coupled into the Y lines by afinger touch. Having narrower Y lines also means that the capacitivecoupling between the X and Y layer is minimized, which, as previouslymentioned, helps to maximize signal-to-noise ratio.

FIG. 7B shows a touch sensor 10 according to an embodiment of theinvention. The sensor 10 shown in the figure combines the electrodepatterns from FIG. 2A and FIG. 7A. The sensor 10 comprises a substrate102 bearing an electrode pattern 30 defining a sensitive area or sensingregion of the sensor and a controller 20. The controller 20 is coupledto electrodes within the electrode pattern by a series of electricalconnections which will be described below. The electrode pattern 30 ismade up of Layer 1 electrodes and Layer 2 electrodes on opposing sidesof the substrate 102 as shown in FIG. 1B.

Referring to FIG. 7B, the controller 20 provides the functionality of adrive unit 12 for supplying drive signals to portions of the electrodepattern 30, a sense unit 14 for sensing signals from other portions ofthe electrode pattern 30, and a processing unit 16 for calculating aposition based on the different sense signals seen for drive signalsapplied to different portions of the electrode pattern. The controller20 thus controls the operation of the drive and sense units, and theprocessing of responses from the sense unit 14 in the processing unit16, in order to determine the position of an object, e.g. a finger orstylus, adjacent the sensor 10. The drive unit 12, sense unit 14 andprocessing unit 16 are shown schematically in FIG. 7B as separateelements within the controller. However, in general the functionality ofall these elements will be provided by a single integrated circuit chip,for example a suitably programmed general purpose microprocessor, orfield programmable gate array, or an application specific integratedcircuit, especially in a microcontroller format.

In the figure there is provided a number of drive electrodes 60represented by longitudinal bars extending in the x-direction asdescribed above and shown in FIG. 2A. On the opposing surface of thesubstrate 102, there is provided a number of sense electrodes 62 formingelectrode Layer 2 as shown in FIG. 7A and described above that cross thedrive electrodes 60 of Layer 1 in the y-direction.

The sense electrodes are then connected to the sense unit 14 viaconnections or tracks 76 and the drive electrodes are connected to thedrive unit 12 via connections or tracks 72. The connections to the driveand sense electrodes are shown schematically in FIG. 7B. However, itwill be appreciated that other techniques for routing the connections ortracks might be used. All of the tracks might be routed to a singleconnector block at the periphery of the substrate 102 for connection tothe controller 20.

The operation of the sensor 10 shown in FIG. 7B is described below. Ascan be seen there are conflicting requirements for the Y lines in termsof their width. The strongest requirement tends to be the minimizationof the resistance of the Y line to ensure successful capacitivemeasurement within an acceptable overall measurement time. This leads towider electrodes, typically in the region of 100 μm to 1000 μm. Wherethe visibility of the electrodes is either not an issue or where theelectrodes can be made practically invisible (such as index matched ITOon PET for example), then the compromises are all quite easilyaccommodated and the width increase is a simple choice. But where thevisibility is an issue and the method used to fabricate the electrodescannot be made sufficiently invisible (such as non index matched ITO onglass) then some alternative arrangement must be found. In this case, amethod called in-filling can be used as now described and illustrated.

FIG. 8A shows a portion of the electrode pattern shown in FIG. 7A withinfilling electrodes. This method fills all “unused” 801 space withisolated squares of conductor 802 (ITO for example), separated with gaps803 to its neighbors that are small enough to be practically invisibleand small enough to cause significant square-to-square capacitance.Another key factor in designing the isolated elements or islands is tomake them the same size 804 in each axis as the width of the Y lines805. In this way, the uniformity of the overall pattern is optimal, andthe only irregularity is in the length of the Y lines. This pattern issubstantially invisible to the human eye. The gaps between neighboringsquares, and the gaps between squares and neighboring Y lines can bemade arbitrarily small, typically in the region of 10's of μm as theyare almost invisible and can be easily mass-produced. The in-filling isgenerated during manufacture at the same time, and using the sameprocess steps, as the sense electrodes, so they are made of the samematerial and have the same thickness and electrical properties as thesense electrodes. This is convenient, but not essential. The in-fillingcould be carried out separately in principle.

The isolated squares 802 serve to obscure the overall pattern but theyalso act as a capacitive interpolator (somewhat analogous to theresistive interpolator used in Layer 1). The capacitive interpolator soformed has the effect of only minimally impacting the fringing fieldsbetween the Y line and the underlying X bars. This is important becausethe field must spread out down to the X bars sufficiently from the edgesof the Y lines to allow a substantial touch influence over at least halfthe pitch of the Y lines. This holds true so long as the capacitancefrom square to square is substantially higher (at least ×2) thecapacitance of a square down to the X bars. The reason for this is thatunder these conditions the electric field tends to propagate from squareto square more easily than it is shunted down to the X layer. As aresult, the field distributions of a design with no in-fill compared toone with in-fill are similar enough that the linearity is preserved. Ifthe square-to-square gaps are increased, the linearity degrades becausethe field tends to pass via the first couple of squares away from a Yline down to the X bars and so does not propagate far from the Y line.

FIG. 8B illustrates these capacitive paths between example infillingelectrodes and between an example infilling electrode and an example Xelectrode. Capacitance from square 808 to square 808 is shown withnominal capacitors 806 and capacitance from one of the squares 808 downto an adjacent X bar 809 is shown with nominal capacitor 807.

It should be noted that the in-fill is not actually needed in thisdesign, but it can be used to minimize pattern visibility withoutcompromising the linearity of the output.

In operation the transmitting or drive electrodes are sequenced suchthat only one driven-X-bar 203 is ever active at one time, all othersbeing driven to a zero potential. The field emitted therefore onlyradiates from one segment at a time. This radiated field couples locallyinto all of the Y lines 701 above the segment in question. The controlchip then takes a capacitive measurement for each of the “intersections”or “crossings” formed between the X and the Y electrodes in thissegment. Each XY intersection is also known as a node. In sequence, eachdriven-X-bar is activated, holding all others at zero potential. In thisway, each segment is sequentially scanned. Once all segments have beencompleted, a total of N×M nodes will have been measured where N is thenumber of driven-X-bars and M is the number of Y lines. It should bestressed that the node measurements are all independent of each othermaking it possible to detect several touch locations simultaneously.Another important point in the way the XY array is scanned is thatbecause only one segment is active at any one time, the others beingdriven to zero potential, only touches in the active segment caninfluence the measured node capacitances in that segment (at least to afirst approximation). This means that an effect known as “hand-shadow”is strongly minimized. Hand-shadow is an effect caused by the proximatelocation of the palm, thumb, wrist etc to the touch screen when the usertouches with a finger.

FIG. 9 shows hand-shadow caused by a proximate location of the palm,thumb, wrist etc to a touch screen when the user touches with a finger.The nature of capacitive measurement means that the electric fields tendto radiate or project from the surface of the device and so can beinfluenced even by objects that are not in direct contact with thesurface. This influence would normally serve to distort the reportedtouch location, as the combined capacitive readings of the fingertogether with readings caused by the “hand shadow” would slightlycorrupt the computed coordinates reported by the control chip. Byactivating only one segment at a time this normally problematic effectis drastically reduced.

Having scanned the entire touch screen, generating N×M nodemeasurements, it is a simple task to compute the touch location, in bothof the axes, of one or more objects, as described in U.S. patentapplication 60/949,376 published as WO 2009/007704[5] on 15 Jan. 2009,using a combination of logical processing to discover the node at theapproximate centre of each touch, and standard mathematical centroidcomputations of the relative signal strengths around each touchdetected. The touch location along the first axis is resolved using thetouch's centre node signal and the immediately adjacent node signal toeach side that lie along the first axis. Likewise, the location in thesecond axis is resolved using the centre node and the immediatelyadjacent node signals that lie along the second axis.

A key design advantage in having the entire Layer 1 almost entirelycovered or flooded with emitting X electrodes is that because theseelectrodes are virtually immune to changes in parasitic capacitiveloading (they are relatively low impendence drivers, even theresistively coupled X bars still only have DC resistances of a few 10'sof KΩ and so can charge and discharge any moderate parasitics veryquickly) any change in the distance between the rear (non-touch side) ofLayer 1 and a nearby ground load will make no difference to the measuredcapacitances of the nodes. The touch screen is thus touch-sensitive onlyon one side, Layer 2. This has major benefits when slightly flexiblefront panels are used that can bend relative to an LCD placed below thetouch screen. The separation between Layer 1 and Layer 2 is fixed by thesubstrate material and hence the capacitance between these two is fixedeven if the substrate is bent during touch causing the rear of Layer 1to experience a change in its ambient conditions.

A further advantage to using the flooded X design is that it provides aninherent amount of noise attenuation for radiated emissions that arepresent behind Layer 1. This is common with LCD modules that tend tohave large amplitude drive signals present on their outer layers. Thesedrive waveforms will normally couple to the Y lines and disturb themomentary reported capacitance of the associated nodes. However, becausethe Y lines are effectively shielded by the flooded X layer, the onlyremaining mechanism for the noise to couple to the Y lines iscapacitively via the X layer itself. The X bars, as already described,are reasonably low resistance and so can only be disturbed by theinterfering noise waveform in proportion to the ratio of the impedanceof the noise coupling vs. the impedance of the X bar. Hence, the amountof noise coupled onward to the Y lines is attenuated by this ratio. Thecoupling of the noise waveform to X bars is purely capacitive and sodecreasing this coupling capacitance helps to attenuate the interferenceeven more. This can be achieved by arranging an air gap between the LCDand the back of Layer 1, or by using a transparent dielectric spacerlayer instead of the air gap that will yield a higher capacitance ofcoupling but has the advantage of being mechanically robust. In atraditional capacitive touch screen an entire extra “shielding” layerbelow Layer 1 must often be used to mitigate this LCD noise. This layeris often driven to zero potential or is actively driven with a facsimileof the capacitive acquisition waveform, which serves to isolate thenoise from the capacitive nodes. This has the disadvantage of addingcost and complexity, worsens optical properties and also tends toattenuate the size of the change in capacitance during touch (leading tolower resolution and worse signal-to-noise ratio). The flooded X designdescribed herein will often produce sufficient inherent attenuation ofthe coupled noise that no extra layer is required, offering asubstantial commercial advantage.

Another advantage found with this design is that the Y lines can be madenarrow in comparison to the size of the touching object. In particular,the Y lines can have a width of one quarter or less than the size of thetouching object, or equivalently the pitch of the X electrodes. Forexample, a Y line width of 0.5 mm is 16 times narrower than the width ofa typical finger touch. The implication of this is related to thesurface area available for interaction with the touching finger. Anarrow Y line has a very small surface area to couple capacitively tothe touch object; in the example cited, the coupled area is around 4 mm²compared with the total “circular” touch area of around 50 mm². Withsuch a small area coupled to the touch, the amount of noise injectedinto the Y line from the finger is minimized because the couplingcapacitance is small. This has an attenuating effect on any differentialnoise between the touch object and device using the touch screen.Furthermore, by making narrow Y lines the resistance is reduced.Reducing the resistance of the Y lines reduces the acquisition times anddecreases the power dissipation.

In summary, the advantages of the described touch screen are:

-   -   1. Only two layers are required for construction leading to; (i)        improved optical transmission (ii) thinner overall        construction (iii) lower cost.    -   2. Area filling design for electrodes on Layer 1 leading to; (i)        almost invisible electrode pattern when using ITO (ii) isolation        of the Y lines on Layer 2 from capacitive effects below Layer        1 (iii) partial attenuation of noise coupled from an underlying        LCD module or other noise source.    -   3. Narrow Y lines on Layer 2 with optional area filling isolated        squares leading to; (i) almost invisible electrode pattern when        using ITO (ii) reduced electrode area reduces susceptibility to        coupling noise from touch.

It is also desirable to minimize the number of Y lines used across Axis1—labeled the first axis in FIG. 7A. This generally leads to a lowercost control chip and simplifies interconnection of the electrodes. Withthe described Y line design, the fundamental pitch between lines needsto be 8 mm or below to achieve good linearity. Spacing the lines furtherapart rapidly compromises linearity in Axis 1. To enable the Y lines tohave a greater “reach” the following adaptation can be made to the Layer2 design.

FIG. 10 shows a portion of an electrode arrangement of sense electrodeswhich modifies the Y line 1101 design to add a series of cross-members1102 running along the first axis 1103 and equally disposed 1104 so asto be centered about the Y line. The cross members span approximately ½to ¾ of the gap to the next Y line 1105 in both directions. The crossmembers on each successive Y line are arranged so that they overlap thecross members of those on the neighboring Y lines 1106 with the gap 1107between the overlapping sections chosen to be a few 10's of μm tominimize visibility and prevent any substantial fringing fields fromforming along the inside of the overlapped region. The cross members arespaced by a distance 1108 along the Y line on a pitch of 8 mm or less,and ideally they are spaced to lie with a uniform relationship to thegaps in the underlying X bars. This ensures that the field patterns areuniform and symmetrical in all regions of the touch screen, leading togood linearity. The cross members effectively act to spread the electricfield further beyond the primary Y line and the overlapped region helpsto gradate the field from one Y region to the next in a linear fashion.

Embodiments of the invention shown in FIGS. 2A, 7A, 7B and 10 mayfurther comprise connections to both extents of the drive and senseelectrodes or transmitting electrodes and Y lines respectively. That isto say that a connection is made at both ends of each of the drive andsense electrodes. This may increase the linearity of the electric filedalong the drive electrodes and improve the shielding of the floodedelectrode design.

Embodiments of the invention may also be applied to non-displayapplications, for example touch pads on a laptop or control panels ondomestic appliances.

FIG. 11 shows a sensor 80 comprising an electrode pattern according toan embodiment of the invention. For simplicity the electrode patternshown in the figure does not include any circuitry. However, it will beappreciated that drive and sense circuitry may also be used as describedabove for the FIG. 7B embodiment. The figure shows an electrode patternon opposing sides of a substrate 82, viewed from above to show therelative position of the electrode patterns.

The electrode pattern comprises two annular electrodes of the typedescribed above referred to as Layer 1 or transmit electrodes. Thetransmit electrodes may also be referred to as drive electrodes. Thedrive electrodes shown in the figure are effectively the transmitelectrodes shown in FIG. 2A and have been wrapped around arcuately toform a complete, or near complete, ring or annulus, as might be used bya scroll wheel sensor for example. Connected to each of the driveelectrodes is a connection or track to provide a drive signal from anappropriate drive unit (not shown). The drive unit described above maybe used. The electrode pattern further comprises a number of senseelectrodes referred to above as Layer 2 electrodes 86 which extendradially from a central point. The Layer 2 electrodes may also bereferred to as sense electrodes or receive electrodes. The senseelectrodes 86 are in the form shown in FIG. 10 and described above. Thesense electrodes are connected to a sense unit (not shown) viaconnections or tracks (not shown). The operation of the sensor 80 issimilar to that described above. However, the readout from a processingunit (not shown) connected to the drive and sense units will bedifferent. The output of the processing unit will provide a polarco-ordinate of an object adjacent the sensor 80. The annular sensor 80may be used in an application where two circular controls are typicallyused in combination, for example the bass and treble controls or theleft/right and front/rear fade controls on a hi-fi amplifier. It will beappreciated that further annular shaped drive electrodes may beimplemented in the sensor 80 shown in the figure. This embodiment maytherefore be summarized as following a polar coordinate grid, with thetwo electrode types extending radially and arcuately, in contrast to theother embodiments which follow a Cartesian coordinate grid, with the twoelectrode types extending along the x- and y-axes.

In a modification of the FIG. 11 design, the arcuate path may extendover a smaller angle for example a quarter or half circle instead of afull circle, or another angular range.

FIG. 12 is a view of a front side of a position sensor 10 according toan embodiment of the invention which follows the design of FIG. 10. Thefront side of the position sensor is typically the side facing the userduring normal use of the sensor or an apparatus incorporating thesensor. The sensor 10 comprises a substrate 40 bearing an electrodepattern 30 defining a sensitive area or sensing region of the sensor anda controller 20. The controller 20 is coupled to electrodes within theelectrode pattern by a series of electrical connections which will bedescribed below. The electrode pattern 30 is on opposing sides of asubstrate, as described below.

The electrode pattern 30 on the substrate 40 can be provided usingconventional techniques (e.g. lithography, deposition, or etch ordeactivation techniques). The substrate is of a dielectric material suchas a plastics film, in this case Polyethylene Terephthalate (PET). Theelectrodes comprising the electrode pattern are of a transparentconductive material, in this case Indium Tin Oxide (ITO). Alternatively,the electrodes could be formed from an opaque conductive material suchas metal e.g. copper. The substrate may be bonded to an overlying panel(not shown) using a suitable pressure sensitive adhesive (PSA) which canbe clear to allow light transmission. Thus the sensitive area of thesensor as a whole is transparent. If transparent, the sensor layer maybe used over an underlying display without obscuration. In otherembodiments, if the sensor layer is opaque, it may comprise aconventional printed circuit board or other substrate with a copperelectrode pattern, e.g. for use in a mobile telephone keypad.

FIG. 13A shows the preferred arrangement of the sensor 10 from a side onview. The figure illustrates how electrodes 60 are disposed on onesurface of the substrate 40 and electrodes 62 are disposed on theopposing surface of the substrate 40. The electrodes 60, 62 are senseand drive electrodes respectively, i.e. the drive electrodes aretypically place farthest away from the touch surface. To isolate thesensor from an object adjacent the sensor an insulating dielectric layer42 is also disposed overlying the sensor. The insulting layer 42 couldbe a glass or plastics panel. The sensor is used to detect the positionof an object adjacent the sensor on the front side 70.

FIG. 13B shows an alternative arrangement of the sensor 10 from a sideon view. The figure illustrates how the electrodes 60 are disposed onone surface of a substrate 40 and the electrodes 62 are disposed on adifferent substrate 44. The two substrates 42, 44 are then broughttogether as shown in the figure. An insulating layer 81 is typicallydisposed between the two sets of electrodes to prevent an electricalcontact between the two set of electrodes. Alternatively, both sets ofelectrodes could be coated with an insulating layer.

FIG. 13C shows an alternative arrangement of the sensor 10 from a sideon view. The figure illustrates how the electrodes 60 are disposed onone surface of a substrate 40. The electrodes 62 are disposed on theelectrodes 60, separated from an insulating layer 81 disposed betweenthe two electrodes patterns 60, 62. Other arrangements are envisagedsuch that the electrode patterns are electrically isolated from oneanother and the electrodes are separated from objects adjacent thesensor by a suitable dielectric material.

Referring to FIG. 12, the controller 20 provides the functionality of adrive unit 12 for supplying drive signals to portions of the electrodepattern 30, a sense unit 14 for sensing signals from other portions ofthe electrode pattern 30, and a processing unit 16 for calculating aposition based on the different sense signals seen for drive signalsapplied to different portions of the electrode pattern. The controller20 thus controls the operation of the drive and sense units, and theprocessing of responses from the sense unit 14 in the processing unit16, in order to determine the position of an object, e.g. a finger orstylus, adjacent the sensor 10. The drive unit 12, sense unit 14 andprocessing unit 16 are shown schematically in FIG. 12 as separateelements within the controller. However, in general the functionality ofall these elements will be provided by a single integrated circuit chip,for example a suitably programmed general purpose microprocessor, orfield programmable gate array, or an application specific integratedcircuit.

In the figure there is provided a number of sense electrodes 62 made upof two elements, herein referred to as spines 64 and branches 66. Thespines 64 of the sense electrodes extend in the y-direction, alsoreferred to as the y direction or a longitudinal direction. The branches66 extend in the x-direction or an x direction crossing the spine 64.The branches 66 extend both left and right from the spines 64, i.e. inboth the negative and positive x-direction in respect of the figure.Negative and positive x-direction are also referred to as the xdirection and a −x direction respectively, wherein the −x directionopposes the x direction. The extent or length of the branches isapproximately ¾ of the spacing between adjacent sense electrode spines64. The branches 66 from adjacent sense electrodes 62 coextend or arecoextensive. That is to say that the branches 66 from adjacent senseelectrode spines 64 occupy the same portion of the sensing region. Theextent of the coextension of the branches 66 shown in FIG. 12 isapproximately ½ of the spacing between adjacent sense electrode spines64. It will be appreciated that the extent of the branches 66 could bevaried as could the extent of the coextension. The spacing between thecoextensive sense electrode branches is typically around ten micrometersor a few tens of micrometers, for example between 5 μm and 50 μm, mostpreferably between 10 μm and 30 μm. The spacing will be chosen toprovide an adequate screening effect while being easy to reliablyfabricate and also invisible to the naked eye of a user.

On the opposing surface of the substrate 40 underlying the senseelectrodes, as described above there is provided a number driveelectrodes 60 represented by longitudinal bars extending in anx-direction also referred to as the x direction or latitudinaldirection. That is to say that the drive electrodes extend along an axisin the x-direction. The drive electrodes 60 are shaded in the figure,but it will be appreciated that these will be constructed from a solidmaterial layer. The drive electrodes 60 are spaced apart by typicallyaround ten micrometers or a few tens of micrometers, for example between5 μm and 50 μm, most preferably between 10 μm and 30 μm. The spacingwill be chosen to provide an adequate screening effect while being easyto reliably fabricate and also invisible to the naked eye of a user.

For a device designed to be actuated by finger touches, the width of thedrive electrodes 60 in the y-direction or the y direction (also referredto as the longitudinal direction), are typically in the range 4-10 mm,for example 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm or 10 mm. The width ofthe drive electrodes is determined by the size of the object that willbe used on the surface of the sensor 10. For example, if the sensor 10is designed to detect the position of a finger adjacent the sensor 10,the width of the drive electrodes 10 will be greater than if the sensor10 is designed to detect the position of a stylus. The area of thesubstrate that is covered by the drive electrodes 60 is may be referredto as the sensing area, the sensing region or the sensitive area.

The drive electrodes shown in FIG. 12 effectively cover or flood thesurface of the substrate with an electrode layer. However, it will beappreciated that the drive electrodes may also be constructed with asmaller width than that shown in the figure and described above, suchthat the each of the drive electrodes 60 covers an area of the senseregion that is similar to that covered by the sense electrodes. That isto say that the drive electrodes might have a width of less than 5 mmand spacing in the y-direction of greater than 30 μm.

The sense electrode branches overlying the drive electrodes aretypically symmetrically disposed over the associated drive electrodes,for example centrally disposed in y between the edges of the associateddrive electrodes. That is to say that the upper and lower edge of a pairof coextensive sense electrode branches is equidistant from the upperand lower edge of the underlying drive electrode. Upper and lower edgesare used to describe the uppermost extent and lowermost extent of adrive electrode in the y-direction.

The drive and sense electrodes shown in FIG. 12 may be considered toform a number of discrete sensing areas, nodes or keys. Referring to theuppermost drive electrode there are seven discrete keys as shown by thedotted regions 68. The different discrete regions are defined by thenumber of exposed edges of the sense electrodes that are present.

It is known that in mutual- or active-type sensing, the majority of thefield lines occur at the edges of the electrodes. Referring to theupper-left discrete key, there is a portion of the branch electrode fromthe leftmost sense electrode spine having two edges, namely an upper andlower edge in the y-direction in the orientation shown. The nextdiscrete region to the right only has one exposed edge from the leftmostsense electrode spine, i.e. the upper edge of the branch electrode. Thisis because the coextension of the branch from an adjacent electrodespine effectively shields the lower edge of this branch. In the nextdiscrete key region to the right there are no exposed edges from theleftmost sense electrode. A similar scenario exists for the second senseelectrode spine. However, as can be seen in the figure this spine willhave two exposed edges at the electrode spine and one exposed edge tothe left and to the right of the electrode spine. The branches spreadthe electric field between adjacent sense electrode spines.

To reduce the number of drive lines used to drive the drive electrodes60, the drive electrodes 60 are coupled via drive channels 72 to thecontroller drive unit 12. Each drive channel supplies drive signals to agroup of three drive electrodes. That is to say that although four driveelectrodes are connected together, the lower drive electrode will beconnected to ground. The drive electrodes 60 are each connected to eachother by a chain or row of resistors 70. Alternatively, a singleresistive strip could be used (not shown in figure). When operated eachof the grouped drive electrodes will receive a different value drivesignal. For example, the drive electrode that is connected directly tothe drive unit 12 will receive the applied signal value, the driveelectrode below will receive two thirds of the applied signal value andthe drive electrode below that will receive a third of the appliedsignal value. In this example the fourth drive electrode connecteddirectly to a drive channel will be connected to ground. However, thisdrive electrode will be driven using the full signal value when the nextgroup of electrodes is driven. It will be appreciated that if there arefewer drive electrodes or there is no restriction on the number ofconnections to the drive unit, the drive electrodes could all be drivenusing separate drive channels. Alternatively, if more drive electrodesare required this can be achieved without introducing more drivechannels by connecting adjacent drive electrodes to each other in groupswith resistors and only directly addressing every second, third, fourthetc. drive electrode.

The sense electrodes are then connected to the sense unit 14 viaconnections or tracks 76 and the drive electrodes are connected to thedrive unit 12 via connections or tracks 72. The connections to the driveand sense electrodes are shown schematically in FIG. 12. However, itwill be appreciated that other techniques for routing the connections ortracks might be used. All of the tracks might be routed to a singleconnector block at the periphery of the substrate 40 for connection tothe controller 20.

The x position of the touch or other actuation is obtained byratiometric interpolation of the signal strength of adjacent senseelectrodes in the x-direction with the highest signal. In FIG. 12 fourelectrodes would be used to interpolate a touch in the x-direction. Oncethe full set of sense signals is collected from driving the driveelectrodes the two adjacent x-electrodes that yielded the strongestsignals are selected, and the x-position determined by ratiometricinterpolation of the signal strength of these two signals.

The y position of the touch or other actuation is also obtained byratiometric interpolation of the signal strength. Once the full set ofsense signals are collected from driving the drive electrodes, the twoy-electrodes that yielded the strongest signals are selected, and the yposition determined by ratiometric interpolation of the signal strengthof these two signals.

Alternative interpolation methods may incorporate weighting factors, forexample one of the known adjacent key suppression algorithms whichsuppress or give lower weighting to signals from some keys, for examplebased on proximity to a key deemed to be a current touch location, orexpected hand shadow effects, as is known in the art. Interpolation neednot be done in a row-wise and column-wise manner as described above. Forexample, interpolation could be among all nearest neighbor keys, or allkeys in a sub-block region previously defined. Many other variations ofinterpolation methods will be envisaged.

FIG. 14A schematically shows a circuit which may be used to measure thecharge transferred from a driven one of the drive electrodes to thesense electrodes, the drive electrode being driven at a given time andthe sense electrode have a self capacitance. This is determinedprimarily by their geometries, particularly in the regions where theyare at their closest. Thus the driven drive electrode is schematicallyshown as a first plate 410 of a capacitor 412 and the sense electrode isschematically shown as a second plate 414 of the capacitor 412.Circuitry of the type shown in FIG. 14B is more fully described in WO00/44018 [1]. The circuit is based in part on the charge-transfer (“QT”)apparatus and methods disclosed in U.S. Pat. No. 5,730,165 [3], thecontents of which are incorporated herein by reference. It will beappreciated that the arrangement of drive electrodes shown in FIG. 12are grouped together. In this case the group of drive electrodes isconsidered to be the driven electrode schematically shown as the firstplate 410. As described above one of the drive connections of a group ofdrive electrodes is connected to a signal and the other drive connectionis connected to ground.

During operation the groups of drive electrodes are sequentially driven,while the sense electrodes are all sensed simultaneously. Alternatively,the sense electrodes could also be sequentially sensed, using eithermany sense units or a single sense unit connected to all the senseelectrodes using appropriate multiplexing.

The drive channel associated with the presently driven electrode 410,the sense channel associated with sense electrode 414 and elements ofthe sensor controller are shown as combined processing circuitry 400 inFIG. 14A. The processing circuitry 400 comprises a sampling switch 401,a charge integrator 402 (shown here as a simple capacitor), an amplifier403 and a reset switch 404, and may also comprise optional chargecancellation means 405.

FIG. 14B shows schematically the timing relationships between the drivenelectrode drive signal from the drive channel 414 and the sample timingof switch 401. The drive channel 414 and the sampling switch 401 areprovided with a suitable synchronizing means, which may be amicroprocessor or other digital controller 408, to maintain thisrelationship. In the implementation shown, the reset switch 404 isinitially closed in order to reset the charge integrator 402 to a knowninitial state (e.g., zero volts). The reset switch 404 is then opened,and at some time thereafter the sampling switch 401 is connected tocharge integrator 402 via terminal 1 of the switch for an intervalduring which the drive channel 414 emits a positive transition, andthereafter reconnects to terminal 0, which is an electrical ground orother suitable reference potential. The drive channel 414 then returnsto ground, and the process repeats again for a total of ‘n’ cycles,(where n may be 1 (i.e. 0 repeats), 2 (1 repeat), 3 (2 repeats) and soon). It can be helpful if the drive signal does not return to groundbefore the charge integrator is disconnected from the sense electrodesince otherwise an equal and opposite charge would flow into/out of thesense channel during positive and negative going edges, thus leading tono net transfer of charge into the charge detector. Following thedesired number of cycles, the sampling switch 401 is held at position 0while the voltage on the charge integrator 402 is measured by ameasurement means 407, which may comprise an amplifier, ADC or othercircuitry as may be appropriate to the application at hand. After themeasurement is taken, the reset switch 404 is closed again, and thecycle is restarted, though with the next drive channel and drivenelectrode in sequence replacing the drive channel 414 and drivenelectrode 100 schematically shown in FIG. 14A. The process of making ameasurement for a given driven electrode is referred to here as being ameasurement ‘burst’ of length ‘n’ where ‘n’ can range from 1 to anyfinite number. The circuit's sensitivity is directly related to ‘n’ andinversely to the value of the charge integrator 402.

It will be understood that the circuit element designated as 402provides a charge integration function that may also be accomplished byother means, and that this type of circuit is not limited to the use ofa ground-referenced capacitor as shown by 402. It should also beself-evident that the charge integrator 402 can be an operationalamplifier based integrator to integrate the charge flowing through inthe sense circuitry. Such integrators also use capacitors to store thecharge. It may be noted that although integrators add circuit complexitythey provide a more ideal summing-junction load for the sense currentsand more dynamic range. If a slow speed integrator is employed, it maybe necessary to use a separate capacitor in the position of 402 totemporarily store the charge at high speed until the integrator canabsorb it in due time, but the value of such a capacitor becomesrelatively non-critical compared to the value of the integrationcapacitor incorporated into the operational amplifier based integrator.

It can be helpful for the sampling switch 401 to connect the senseelectrode of the sensor to ground when not connected to the chargeintegrator 402 during the changes of drive signal of the chosen polarity(in this case positive going). This is because this can create anartificial ground plane, thus reducing RF emissions, and also, as notedabove, permitting the coupled charge of opposite polarity to that beingsensed by the charge integrator 402 to properly dissipate andneutralize. It is also possible to use a resistor to ground on the senseelectrode to accomplish the same effect between transitions of drivechannels 414. As an alternative to a single-pole double-throw (SPDT)switch 401, two independent switches can be used if timed in anappropriate manner.

As described in U.S. Pat. No. 5,730,165 [3], there are many signalprocessing options possible for the manipulation and determination of adetection or measurement of signal amplitude. U.S. Pat. No. 5,730,165[3] also describes the gain relationship of the arrangement depicted inFIG. 12, albeit in terms of a single electrode system. The gainrelationship in the present case is the same. The utility of a signalcancellation means 405 is described in U.S. Pat. No. 4,879,461 [4], aswell as in U.S. Pat. No. 5,730,165 [3]. The disclosure of U.S. Pat. No.4,879,461 [4] is herein incorporated by reference. The purpose of signalcancellation is to reduce the voltage (i.e. charge) build-up on thecharge integrator 402 concurrently with the generation of each burst(positive going transition of the drive channel), so as to permit ahigher coupling between the driven electrodes and the receiving senseelectrodes. One benefit of this approach is to allow a large sensingarea that is sensitive to small deviations in coupling between theelectrodes at relatively low cost. Such large sense couplings arepresent in physically relatively large electrodes such as might be usedin human touch sensing pads. Charge cancellation permits measurement ofthe amount of coupling with greater linearity, because linearity isdependent on the ability of the coupled charge from the driven electrode100 to the sense electrode 104 to be sunk into a ‘virtual ground’ nodeover the course of a burst. If the voltage on the charge integrator 402were allowed to rise appreciably during the course of a burst, thevoltage would rise in inverse exponential fashion. This exponentialcomponent has a deleterious effect on linearity and hence on availabledynamic range.

The drive channel may be a simple CMOS logic gate powered from aconventionally regulated supply and controlled by the sensor controller20 to provide a periodic plurality of voltage pulses of a selectedduration (or in a simple implementation a single transition fromlow-to-high or high-to-low voltage, i.e. a burst of one pulse).Alternatively, the drive channel may comprise a sinusoidal generator orgenerator of a cyclical voltage having another suitable waveform. Achanging electric field is thus generated on the rising and failingedges of the train of voltage cycles applied to the driven electrode.The driven electrode and the sense electrode are assumed to act asopposing plates of a capacitor having a capacitance C_(E). Because thesense electrode is capacitively coupled to the driven electrode, itreceives or sinks the changing electric field generated by the drivencolumn electrode. This results in a current flow in the sense electrodeinduced by the changing voltage on the driven electrode throughcapacitive differentiation of the changing electric fields. The currentwill flow towards (or from, depending on polarity) the sense channels inthe sense unit 14. As noted above, the sense channel may comprise acharge measurement circuit configured to measure the flow of chargeinto/out of (depending on polarity) the sense channel caused by thecurrents induced in the sense electrode.

The capacitive differentiation occurs through the equation governingcurrent flow through a capacitor, namely:

$I_{E} = {C_{E} \times \frac{V}{t}}$

where I_(E) is the instantaneous current flowing to a sense channel anddV/dt is the rate of change of voltage applies to a driven electrode.The amount of charge coupled to the sense electrode (and so into/out ofthe sense channel) during an edge transition is the integral of theabove equation over time, i.e.

Q _(E) =C _(E) ×V.

The charge coupled on each transition, Q_(E), is independent of the risetime of V (i.e. dV/dt) and depends only on the voltage swing at thedriven electrode (which may readily be fixed) and the magnitude of thecoupling capacitance C_(E) between the driven electrode and senseelectrode. Thus a determination of the charge coupled into/out of chargedetector comprising the sense channel in response to changes in thedrive signal applied to the driven electrode is a measure of thecoupling capacitance C_(E) between the driven electrode and the senseelectrode.

The capacitance of a conventional parallel plate capacitor is almostindependent of the electrical properties of the region outside of thespace between the plates (at least for plates that are large in extentcompared to their separation). However, for a capacitor comprisingneighboring electrodes in a plane this is not the case. This is becauseat least some of the electric fields connecting between the drivenelectrode and the sense electrode “spill” out from the substrate. Thismeans the capacitive coupling (i.e. the magnitude of C_(E)) between thedriven electrode and the sense electrode is to some extent sensitive tothe electrical properties of the region in the vicinity of theelectrodes in to which the “spilled” electric field extends.

In the absence of any adjacent objects, the magnitude of C_(E) isdetermined primarily by the geometry of the electrodes, and thethickness and dielectric constant of the sensor substrate. However, ifan object is present in the region into which the electric field spillsoutside of the substrate, the electric field in this region may bemodified by the electrical properties of the object. This causes thecapacitive coupling between the electrodes to change, and thus themeasured charge coupled into/from the charge detector comprising thesense channel(s) changes. For example, if a user places a finger in theregion of space occupied by some of the of the spilled electric fields,the capacitive coupling of charge between the electrodes will be reducedbecause the user will have a substantial capacitance to ground (or othernearby structures whose path will complete to the ground referencepotential of the circuitry controlling the sense elements). This reducedcoupling occurs because the spilled electric field which is normallycoupled between the driven electrode and sense electrode is in partdiverted away from the electrode to earth. This is because the objectadjacent the sensor acts to shunt electric fields away from the directcoupling between the electrodes.

Thus by monitoring the amount of charge coupled between the drivenelectrode and the sense electrode, changes in the amount of chargecoupled between them can be identified and used to determine if anobject is adjacent the sensor (i.e. whether the electrical properties ofthe region into which the spilled electric fields extend have changed).

FIG. 15A, FIG. 16 and FIG. 17 show further electrode patterns embodyingthe invention which can be applied to a substrate incorporated in acapacitive position sensor. The electrode patterns of these furtherembodiments are extensions of the y-interpolation approach taken by thepattern of FIG. 12 in which multiple branches of each sense electrodeare provided for each drive electrode. The same reference numerals areused to denote corresponding features where appropriate.

FIG. 15A is a view of a front side of a position sensor 10 according toanother embodiment of the invention. The position sensor shown in FIG.15A is similar to the sensor shown in FIG. 12 in layout and operationexcept the number of branches 66 that extend from the sense electrodepine 64 is different. In the sensor shown in the figure there are twobranches that extend from the spine 64 at each drive electrode 60location. The length of the two branches is different and thecoextension is also offset from one another at the location of eachdrive electrode 60. This is done to alter the number of exposed edges ofthe branches of the sense electrodes. The two individual branches ofeach of the two sets of branches extend ⅜'ths and ⅞'ths of the way fromtheir own spine to the adjacent sense electrode spine 64 to provide anoverlap of ⅞+⅜−1=⅛=¼ of the spine separation distance. As can be seen inthe figure there are four sense electrode spines 64 and thus four sensechannels 76 (same as FIG. 12), but the number of discrete sensingelements or regions 68 is thirteen (compared with 7 in FIG. 12).

FIG. 15B shows an area 92 of the position sensor 10 shown in FIG. 15A asan expanded view. The expanded view of FIG. 15B shows two adjacentspines 1502, 1504. There are four branches 1506 extending from thespines, two from each spine 1502, 1504. The exposed edges are the edgesof the branches that are not adjacent another branch electrode. Forexample, at the left spine shown in the figure there are 4 exposed edges1508, 1510, 1512, 1514. The number of exposed branch edges for the leftelectrode spine 1502 shown in the figure is 4 at the left, anddecrements by 1 for each discrete sensing element or region defined by adotted square in the figure. At the adjacent spine 1504 on the right,there are no exposed edges from the left spine 1502. The number ofexposed edges of the left spine 1502 at each region is marked on FIG.15B along the upper edge of the expanded area. Similarly the number ofexposed branch edges of the right spine 1504 at each region is marked onFIG. 15B along the lower edge of the expanded area. The extent orboundary of the flooded x-electrode in the y-direction is shown in theFigure by two horizontal dotted lines 1516.

FIG. 16 is a view of a front side of a position sensor 10 according toanother embodiment of the invention. The position sensor shown in FIG.15 is similar to the sensor shown in FIG. 12 in layout and operationexcept the number of branches 66 that extend from the sense electrodespine 64 is increased. In the sensor shown in the figure there are threebranches that extend from the spine 64 at each drive electrode 60location. The length of the three branches is different and thecoextension region is also offset from one another at each driveelectrode 60 location. This is done to alter the number of exposed edgesof the branches. The three branches in the figure that extend in the xdirection or right from the sensor spines extend to 7/12, 9/12 and 11/12respectively of the spacing between adjacent sense electrode spines 64.The three branches that extend in the −x direction or left from thesensor spines extend 11/12, 9/12 and 7/12 respectively of the spacingbetween adjacent sense electrode spines 64. As can be clearly seen fromthe figure there are still only four sense electrode spines 64 and thusfour sense channels 76, but the number of discrete sensing elements 68has increased from 7 in FIG. 12 to 20. Referring to FIG. 16, the numberof exposed branch edges starting from the left-most electrode spine 64is 6, and decrements by 1 for each discrete sensing element 68 down tozero at the adjacent sensor electrode spine.

FIG. 17 is a view of a front side of a position sensor 10 according toanother embodiment of the invention. The position sensor shown in FIG.18 is similar to the sensor shown in FIG. 12 in layout and operation.However, the position sensor shown in the figure has an alternativearrangement of electrodes. The drive and sense electrodes shown in thefigure are made up of thin wires or a mesh of wire instead of thecontinuous layer of electrode material shown in FIG. 12. The driveelectrodes 60 are constructed by a rectangular perimeter of wire todefine the shape of the drive electrode with a series of diagonal wirelines going across the rectangular perimeter. The diagonal lines aretypically at an angle of 45 degrees to an axis running along thex-direction. The diagonal lines and the rectangular perimeter of eachdrive electrode are electrically connected and connected to the driveunit 12 via the drive channels 72. The wires or mesh are manufacturedfrom metal wires e.g. copper, but could also be made of gold or silver.Similarly the sense electrodes are also manufactured using a thin metaltrace that follows the perimeter of the sense electrode pattern shown inFIG. 12. The sense electrodes 62 are relative narrow compared to thedrive electrodes 60, so there is no need to use in-filling with diagonallines. However, some extra wires are added within the sense electrodemesh structure as shown in FIG. 17 by short lines 78. This is to addredundancy in the pattern, so that if there is a defect in the electrodewire at one location, the current has an alternative path. Such defectscan occur if there is a defect in the optical mask used to pattern thewires or if there is debris on the surface of the wires duringprocessing. It will be appreciated that the electrode arrangements shownin FIG. 16 and FIG. 17 may also be constructed from the electrode wiresor mesh as described above.

It will be understood that the “mesh” or “filligrane” approach toforming each electrode out of a plurality of interconnected fine linesof highly conducting wire or traces may be used for either Layer 1 (Xdrive), Layer 2 (Y sense) or both. The FIG. 17 embodiment uses meshesfor both layers. However, a particularly preferred combination fordisplay applications or other applications where invisibility isimportant is that Layer 1 is made with non-mesh, i.e. “solid” electrodeswith the small, invisible gaps, for example from ITO, and Layer 2 ismade with mesh electrodes, for example out of copper, having line widthssufficiently small to be invisible also.

It will be appreciate that the patterns shown in FIG. 12, FIG. 15, FIG.16 and FIG. 17 can be repeated or extended in both the x- andy-direction.

The sensors shown in FIG. 12, FIG. 15, FIG. 16 and FIG. 17 allow thewidth (x-direction) of a 2DCT to be increased without increasing thenumber of sense channels, while retaining the same resolution and usinga linear interpolation technique. This is clearly shown in FIG. 12, FIG.15 and FIG. 16 which although schematic and not to scale in an absolutesense are drawn to the same scale as each other to illustrate how thetotal width of the 2DCT can be increased by increasing the number ofy-interpolation branches per drive “cell” without changing the number ofsense channels which is four in all the illustrated examples.

The number of sense channels can vary. Moreover, the number of sensechannel branches per drive cell can be greater than illustrated above.The above embodiments show 1, 2 and 3 branches per drive cell, but thenumber could be 4, 5, 6 or more in principle, although there is likelyto be a practical limit that arises from usual design trade-offs betweennumbers of channels, read-out time and complexity of the electrodepatterns.

The role of the branches is therefore to linearize the transition fromone sense channel to the next.

Generally the number of “keys” ‘K’ will be a function of the number ofsense channels ‘n’ and the number of sense branches per drive electrodecell ‘m’ given by the formula K=2m(n−1)+1. Tabulated for values of ‘m’up to 4 and ‘n’ up to five, this gives the following numbers of keys:

n = 2 n = 3 n = 4 n = 5 m = 1 3 5 7 9 m = 2 5 9 13 17 m = 3 7 13 19 25 m= 4 9 17 25 33

It will also be understood that the drive electrode spacing willpreferably have comparable dimension to the touch size of the touchingobject for which the sensor is designed. On the other hand, through theuse of the y-interpolation feature of the sense electrode branches, thesense electrode spacing can be sparser. With only one set of senseelectrode branches per drive electrode (FIG. 12), the sense electrodespacing will generally be twice the drive electrode spacing assuming thekey dimensions in x- and y- are set the same. With two sets of senseelectrode branches per drive electrode (FIG. 15), the sense electrodespacing will generally be four times the drive electrode spacing. Withthree sets of sense electrode branches per drive electrode (FIG. 16),the sense electrode spacing will generally be six times the driveelectrode spacing. That is to say, the sense electrode spacing will ingeneral be ‘2m’ times the drive electrode spacing assuming the keydimensions in x and y are set the same. Normally the key dimensions in xand y if not the same will be similar, so the sense electrode spacingwill usually be within ‘2m’ times the drive electrode spacing. Further,if the touching object for which the sensor is designed is a humanfinger, then each key will typically have x- and y-dimensions in therange 5-10 mm.

In the above, it will be understood that reference to ‘per driveelectrode’ relates to individual drive electrodes regardless of whetherthe drive electrodes are directly addressed or not. For example, in theembodiment of FIG. 12, there are 10 drive electrodes, but only 4 drivechannels as a result of use of bridging resistors 74.

It will be appreciated that the sensor of the invention is applicable tomany types of device/appliance. For example, sensors can be used withovens, grills, washing machines, tumble-dryers, dish-washers, microwaveovens, food blenders, bread makers, drinks machines, computers, homeaudiovisual equipment, personal computers, portable media players, PDAs,cell phones, computers, games consoles and so forth.

FIG. 18 shows an example of a mobile personal computer (PC) 120. A touchsensor according to the present technique could be used to form part orthe whole of an input control panel of the notebook PC 120. In thefigure, the PC 120 is shown, which includes a display device 122attached to a base 124, which accommodates a processor and othercomponents typically associated with a PC. An input control panel 126includes a keyboard 128. The input control panel 126 further includes atouch sensitive mouse pad 130. The mouse pad can be implemented using atouch sensor according to the present invention. Moreover, the displaydevice 122 can also be implemented with a touch sensor according to thepresent invention overlaid on top of it to provide a touch screen. Thismay be particularly useful for a tablet PC.

FIG. 19 schematically shows a washing machine 91 incorporating a controlpanel 93 which incorporates a sensor according to the invention.

FIG. 20 schematically shows a cellular telephone 95 which mayincorporate one or more sensors according to an embodiment of theinvention. A two-dimensional sensor 98 according to the invention may beused to provide the button panel with buttons 99, or may be a separatesensor co-extensive with the button panel. For example, the button panelmay be retained as a mechanical assembly and the sensor provided toallow drawing, writing or command gestures to be performed by the userover the button panel area, for example to compose text messages inChinese or other Asian characters. The screen 97 may also be overlaidwith a sensor according to the invention.

More generally the invention may be used in conjunction with anyappliance having a human-machine interface. It is also possible toprovide a sensor similar to those described above which is providedseparately from a device/appliance which it may be used to control, forexample to provide an upgrade to a pre-existing appliance. It is alsopossible to provide a generic sensor which may be configured to operatea range of different appliances. For example, a sensor may be providedthat has programmable keys which a device/appliance provider mayassociate with desired functions by appropriately configuration, forexample by reprogramming.

1. A capacitive position sensor comprising: a plurality of driveelectrodes extending in a first direction on a first plane; a pluralityof sense electrodes extending in a second direction on a second planeoffset from the first plane so that the sense electrodes cross the driveelectrodes at a plurality of intersections which collectively form aposition sensing array; wherein the sense electrodes have branchesextending in the first direction part of the way towards each adjacentsense electrode so that end portions of the branches of adjacent senseelectrodes co-extend with each other in the first direction separated bya distance sufficiently small that capacitive coupling to the driveelectrode adjacent to the co-extending portion is reduced.
 2. The sensorof claim 1, wherein for each drive electrode there is one set of senseelectrode branches providing co-extending portions that occupy a regionin between adjacent sense electrodes in the first direction.
 3. Thesensor of claim 1, wherein for each drive electrode there are multiplesets of sense electrode branches that are offset from each other in thesecond direction, the multiple sets providing respective co-extendingportions extending over different respective regions in the firstdirection.
 4. The sensor of claim 3, wherein the sense electrodes areseparated from each other in the first direction by a distance P_(sense)and the drive electrodes are separated from each other in the seconddirection by a distance P_(drive), wherein:P _(sense) /P _(drive)=2m±1 where ‘m’ is the number of sets of senseelectrode branches per drive electrode.
 5. The sensor of claim 4,wherein P_(drive) is of comparable dimension to the touch size of thetouching object for which the sensor is designed.
 6. The sensor of claim1, wherein in plan view each drive electrode covers an area that fullyencloses its associated sense electrode branches.
 7. The sensor of claim1, wherein the gaps between individual ones of the drive electrodes aresmall.
 8. The capacitive touch sensor of claim 7, wherein the gaps aredimensioned to be sufficiently small to be invisible or almostinvisible.
 9. The capacitive touch sensor of claim 7, wherein the gapsare less than around 100 micrometers.
 10. The capacitive touch sensor ofclaim 1, further comprising a display module arranged below the driveelectrodes.
 11. The capacitive touch sensor of claim 10, wherein thedisplay module is an LCD.
 12. The capacitive touch sensor of claim 1,wherein each drive and/or sense electrode is made of a continuous sheetof electrically conductive material.
 13. The capacitive touch sensor ofclaim 1, wherein each drive and/or sense electrode is made of a mesh orfiligree pattern of interconnected lines of highly conductive materialwhich collectively define each electrode.
 14. The capacitive touchsensor of claim 13, wherein the interconnected lines have a sufficientlysmall width so as to be invisible or almost invisible.
 15. Thecapacitive touch sensor of claim 1, wherein the drive electrodes extendin a first linear direction and the sense electrodes extend in a secondlinear direction transverse to the first linear direction so that theplurality of intersections form a grid pattern.
 16. The capacitive touchsensor of claim 1, wherein the drive electrodes extend arcuately and thesense electrodes extend radially so that the plurality of intersectionslie on one or more arcuate paths.
 17. A touch sensitive panel for acapacitive touch sensor, the touch sensitive panel having a plurality ofdrive electrodes arranged on one side of a substrate in a first layerand a plurality of sense electrodes arranged on the other side of thesubstrate in a second layer so that the sense electrodes cross the driveelectrodes at a plurality of intersections offset from each other by thethickness of the substrate, wherein the sense electrodes have branchesextending in the first direction part of the way towards each adjacentsense electrode so that end portions of the branches of adjacent senseelectrodes co-extend with each other in the first direction separated bya distance sufficiently small that capacitive coupling to the driveelectrode adjacent to the co-extending portion is reduced.
 18. A methodof sensing position of an actuation on a two-dimensional position sensoraccording to claim 1, the method comprising: applying drive signals toeach of the plurality of drive electrodes; measuring sense signalsreceived from each of the plurality of sense electrodes representing adegree of capacitive coupling of the drive signals between the driveelectrodes and each group of the sense electrodes; determining theposition in the first direction by an interpolation between sensesignals obtained from each of the plurality of sense electrodes; anddetermining the position in the second direction by an interpolationbetween sense signals obtained by sequentially driving each of theplurality of drive electrodes with respective drive signals.